Add warning flags to CMake list file

Settle on a CMake minimum version of 3.16, which is the version in
Ubuntu 20.04. It includes the change to MSVC default compiler flags
so we can set our own warning level.

CMakeLists.txt

@@ -1,9 +1,15 @@
-cmake_minimum_required(VERSION 3.21)
+cmake_minimum_required(VERSION 3.16)
 
 project(parg C)
 
 include(CTest)
 
+if(MSVC)
+  add_compile_options(/W3)
+else()
+  add_compile_options(-Wall -Wextra -pedantic)
+endif()
+
 add_library(parg parg.c parg.h)
 target_include_directories(parg PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>)